A popcorn model for radioisotopic dating

Contents

Learning activity

The dark band in this picture is a layer of
volcanic ash trapped in ice from the East Antarctic ice sheet. The
ash was deposited on snow on the surface of the ice sheet, which was
then compressed over time to form ice. Determining the age of the
ash layer will reveal the age of the surrounding ice.

Geologists take advantage of decay of natural radioactive elements to determine
the age of rocks, which can help us understand earth history. One example of
this dating method is using volcanic ash layers to help determine the age of
ash-bearing horizons in ice cores. Ice cores provide important records of past
climate conditions because the chemical composition of the ice reveals past temperature,
and tiny bubbles of air trapped between ice crystals can reveal past atmospheric
composition. The volcanic ash can be dated using a technique called Potassium-Argon
(K-Ar) geochronology.

K-Ar geochronology relies on the principal that radioactive elements
decay over time, with a parent isotope of potassium (K) decaying to form
the daughter isotope argon, (Ar). The daughter isotope argon does not start
to accumulate until a volcanic eruption takes place, at which point the
radioactive decay clock starts. Over a very long period of time, almost
all of the radioactive K will decay to form Ar. The time that it takes
for half of the isotope of K to form Ar is called the “half life,” which
in the case of K-Ar is 1.25 billion years. By measuring the ratio of K
to Ar in feldspar crystals in ash from a volcanic eruption, the age of
the eruption, and therefore the age of the ice in which the ash is found,
can be determined.

A simple way to simulate radioactive decay is by making popcorn. Popcorn
starts out as unpopped “parent” kernels we’ll call “kernelite, (Ke).” Heating
starts the radioactive decay clock and the “kernelite” begins to decay
to a new daughter product of popped kernels we’ll call “popcornium, (Pc).”
Just like radioactive decay, this process is irreversible, and with enough
time all the kernelite will decay to popcornium. The “half-life” of kernelite
is the time after which half of the kernels have popped, transforming to
popcornium.

The following experiment using the “kernelite/popcornium” system can
help understand radioactive decay. After acquiring the data, the next
step is to plot the “decay” curve of kernelite, and the “accumulation”
curve of popcornium. Next use these curves first to establish the “half-life”
of kernelite, and second to determine the “age” (popping time) for bags
of popcorn for which the age is unknown. The experiment involves several
steps described below.

Procedure

Using a microwave oven, pop 6 bags of popcorn one at a time.Label
six of the bags with predetermined popping times t=0 sec, 10 sec, 20
sec, 30 sec, 40 sec, and 50 sec. Preset the microwave time for 2 minutes.
Even though you will only be popping for a short time, some time will
elapse as popcorn heats up and begins popping. The “radioactive
decay” timing begins when you hear the first kernel of popcorn pop.
Use the microwave timer or a stopwatch to measure the popping time. Turn
the microwave off and remove the bag as soon as the predetermined (decay)
popping time is reached.

Label the remaining bags A, B, C, and D. Pop each for time intervals
between 10 and 50 seconds. Secretly record the time for each of the
bags. These are the unknown samples for which the “age” (popping time)
will be determined.

Divide the class into 10 groups. Each group will open a bag of popped
corn, spread the contents on a large sheet of parchment paper and carefully
count and record the number of a) kernelite b) popcornium kernels. Record
your results on the data sheet.

Plot the results from bags with known popping time intervals on the
graph with time (T) on the horizontal axis, % kernelite (parent) ratio
on the vertical axis (0-100% scale). On the same graph, also plot the
% popcornium (daughter). You can now determine the “half-life” of the
Ke/Pc system by finding the point where both % Ke and % Pc are 50% (where
the two plotted lines cross) and reading the time from the horizontal
axis.

Find the % Ke values from “unknown” bags (A-D) and plot where they
intersect your decay curve. Now, determine the unknown “age” (popping
time) for each bag by reading the time from the X-axis that corresponds
to the measured % Ke. Compare your results to the mystery popping
times that were secretly recorded.

Discuss the ways in which experimental errors can affect your results.
How might your experimental popcornium/kernelite decay system differ
from a natural radioactive decay process, such as occurs in volcanic
ash layers in ice cores? How else might scientists use radioisotopic
dating to study climate history and other geologic records?

Ice coring on the summit of Mount Moulton, in West Antarctica. A portable
“Eclipse” drill rig is shown, and scienctists hold a segment of ice core retrieved
by the drilling operation. A thick ash layer is shown in the background (dark
band) and the source volcano, Mt. Berlin, is in the far distance.